NON-POLAR OR SEMI-POLAR GaN WAFER

ABSTRACT

A non-polar or semi-polar GaN wafer in which a lower-crystallinity band present on a main surface has a reduced width. The GaN wafer includes a first main surface and a second main surface on a side opposite to the first main surface. The first main surface is parallel to or tilted relative to the M-plane. When the tilt, if exists, is decomposed into the a-axis direction component and the c-axis direction component, the a-axis direction component has an absolute value of 5° or less while the c-axis direction component has an absolute value of 45° or less. The GaN wafer includes a lower-crystallinity band extending on the first main surface in a direction perpendicular to the c-axis, and the lower-crystallinity band has a width of less than 190 μm.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of International Application PCT/JP2016/064969,filed on May 20, 2016, and designated the U.S., and claims priority fromJapanese Patent Application 2015-140701 which was filed on Jul. 14,2015, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a non-polar or semi-polar GaN (galliumnitride) wafer. A GaN wafer herein refers to a freestanding GaN waferformed only of GaN, unless otherwise noted.

BACKGROUND ART

GaN is one of group III-V compound semiconductors and has a wurtzitecrystal structure belonging to hexagonal crystal systems.

By doping with an impurity, GaN can be made conductive. Examples ofknown n-type impurities include O (oxygen), Si (silicon), and Ge(germanium). Examples of known p-type impurities include Mg (magnesium)and Zn (zinc). Some impurities exemplified by Fe (iron) are used to makeGaN into an insulator having a high resistance.

A non-polar or semi-polar GaN wafer having a main surface with a largetilt relative to the C-plane is promising as a substrate for nitridesemiconductor devices with improved characteristics (Non-Patent Document1). Nitride semiconductors are also referred to as, for example,nitride-based Group III-V compound semiconductors, group III nitridecompound semiconductors, and GaN-based semiconductors, and include, inaddition to GaN, a compound in which Ga in GaN is partially or fullysubstituted with another Group 13 element (B, Al, and In) in theperiodic table. Examples of such a compound include AlN, InN, AlGaN,AlInN, GaInN, and AlGaInN.

Among non-polar GaN wafers, (10-10) wafers, namely M-plane wafers haveespecially attracted attention. Among semi-polar GaN wafers, (20-21)wafers, (20-2-1) wafers, (30-31) wafers, and (30-3-1) wafers haveespecially attracted attention.

A non-polar or semi-polar GaN wafer can be produced by a method in whicha bulk GaN crystal grown on the GaN (0001) surface of a C-plane GaNwafer or C-plane GaN template by a Hydride Vapor Phase Epitaxy (HVPE)method is sliced parallel to a desired non-polar or semi-polar plane.However, the thickness of a GaN crystal stably growable on the GaN(0001) surface by vapor phase epitaxy is usually several millimeters orless; therefore, a non-polar or semi-polar GaN wafer made by this methodhas a limited area. It is extremely difficult to industriallymanufacture large-area wafers such as a 2-inch wafer (a disk-shapedwafer having a diameter of about 50 mm) by this method.

To solve this problem, a tiling method has been devised. In the tilingmethod, an aggregated seed is used. The aggregated seed is formed byclosely arranging side by side a plurality of seeds in a manner to haveuniform crystal orientation. On the aggregated seed formed of theplurality of seeds, a bulk GaN crystal forming a single continuous layeris epitaxially grown by an HVPE method (Patent Documents 1 to 4). Byusing an aggregated seed formed by gathering a plurality of M-plane GaNwafers each having a main surface with a size of as small as severalmillimeters in the c-axis direction, an M-plane GaN wafer having adiameter of about 50 mm is able to be realized.

An ammonothermal method is known as one of methods enabling growth of ahigh-quality GaN crystal with less defects such as dislocation (PatentDocument 5). In the ammonothermal method, ammonia in a supercritical orsubcritical state is used as a solvent to precipitate a GaN singlecrystal on a seed.

PRIOR ART DOCUMENT Patent Document

Patent Document 1: JP-A-2006-315947

Patent Document 2: JP-A-2008-143772

Patent Document 3: JP-A-2010-275171

Patent Document 4: JP-A-2011-026181

Patent Document 5: WO2014/129544

Non-Patent Document

Non-Patent Document 1: Po Shan Hsu, Matthew T. Hardy, Erin C. Young,Alexey E. Romanov, Steven P. DenBaars, Shuji Nakamura, and James S.Speck, Applied Physics Letters 100, 171917 (2012)

SUMMARY OF INVENTION Problem to be Solved by Invention

In a bulk GaN crystal grown by a tiling method, a portion grown abovethe boundary between adjacent seeds has lower crystallinity. A non-polaror semi-polar GaN wafer produced by slicing such a GaN crystal has, on amain surface thereof, a lower-crystallinity band which is an area wherethe above-described portion having lower crystallinity is exposed in aband-shape.

When the non-polar or semi-polar GaN wafer having thelower-crystallinity band on a main surface thereof is used to produce anitride semiconductor device, an element formed right above thelower-crystallinity band would be inferior to another element formed onother portion of the same wafer in terms of characteristics andreliability. Thus, if a reduction in the width of thelower-crystallinity band is achieved, yields of nitride semiconductordevices produced by using the non-polar or semi-polar GaN wafer would beimproved.

Accordingly, it is a main object of the present invention to provide anon-polar or semi-polar GaN wafer in which a lower-crystallinity bandpresent on a main surface has a reduced width.

Solution to Problem

Aspects of the present invention include a GaN wafer described below.

-   (1) A GaN wafer including: a first main surface; a second main    surface on a side opposite to the first main surface; and a    lower-crystallinity band extending on the first main surface in a    direction perpendicular to a c-axis, wherein the first main surface    is parallel to or tilted relative to an M-plane, when the tilt, if    exists, is decomposed into an a-axis direction component and a    c-axis direction component, the a-axis direction component has an    absolute value of 5° or less while the c-axis direction component    has an absolute value of 45° or less, and the lower-crystallinity    band has a width of less than 190 μm.-   (2) The GaN wafer according to (1), wherein the lower-crystallinity    band has a width of less than 150 μm.-   (3) The GaN wafer according to (2), wherein the lower-crystallinity    band has a width of less than 120 μm.-   (4) The GaN wafer according to any one of (1) to (3), wherein X-ray    rocking curve full width at half maximums of a (100) plane measured    in an area distant from an outer periphery by 3 mm or more on the    first main surface are less than 0.01°, except in the    lower-crystallinity band.-   (5) The GaN wafer according to any one of (1) to (4), wherein the    GaN wafer is a {10-10} wafer, a {10-11} wafer, a {10-1-1} wafer, a    {20-21} wafer, a {20-2-1} wafer, a {30-31} wafer or a {30-3-1}    wafer.-   (6) The GaN wafer according to any one of (1) to (5), wherein the    GaN wafer is a disk having a diameter of from 45 to 55 mm, and the    number of the lower-crystallinity band the GaN wafer has on the    first main surface is one or greater and three or smaller.-   (7) The GaN wafer according to any one of (1) to (5), further    including a lower-crystallinity band extending on the first main    surface in a direction perpendicular to an a-axis.-   (8) The GaN wafer according to (7), wherein the GaN wafer is a disk    having a diameter of from 95 to 155 mm.-   (9) A method for producing a nitride semiconductor device,    including: preparing the GaN wafer according to any one of (1) to    (8); and growing one or more nitride semiconductors on the prepared    GaN wafer by vapor phase epitaxy to form a device structure.

Effect of the Invention

A non-polar or semi-polar GaN wafer in which a lower-crystallinity bandpresent on a main surface has a reduced width is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating an example of an M-plane GaNwafer according to a first embodiment.

FIG. 2A is a plan view of the M-plane GaN wafer illustrated in FIG. 1,and FIG. 2B is a cross-sectional view of the M-plane GaN waferillustrated in FIG. 1.

FIG. 3 is a plan view illustrating one example of the M-plane GaN waferaccording to the first embodiment.

FIG. 4A is a cross-sectional view illustrating a state in which aplurality of tiling GaN seeds are adhered to a surface of a flat platewhile being closely arranged side by side, and FIG. 4B is across-sectional view illustrating a state after the plurality of tilingGaN seeds illustrated in FIG. 4A are subjected to planarizationprocessing.

FIG. 5 illustrates a cross-sectional view of a composite of anaggregated seed and a bulk GaN crystal epitaxially grown on theaggregated seed.

FIG. 6 is a cross-sectional view illustrating a state after a pluralityof tiling GaN seeds have been adhered to a flat surface of a plate whilebeing spaced apart from each other and have been subjected toplanarization processing.

FIG. 7 illustrates a cross-sectional view of a composite of anaggregated seed and a bulk GaN crystal epitaxially grown on theaggregated seed.

FIG. 8 is a cross sectional-view of the composite illustrated in FIG. 5after being processed to have a cylindrical shape, and broken linesindicate positions of slicing when slicing the composite to obtain theGaN wafer illustrated in FIG. 1.

FIGS. 9 A to 9C are graphs each showing the result of measurement of anXRC-FWHM of the (100) plane at about every 27 μm on a measurement lineextending on a main surface of an M-plane GaN wafer of an Example in thec-axis direction.

FIGS. 10A to 10C are graphs each showing the result of measurement of anXRC-FWHM of the (100) plane at about every 27 μm on a measurement lineextending on a main surface of an M-plane GaN wafer of a ComparativeExample in the c-axis direction.

FIG. 11 is a drawing illustrating the relationship between a tilt θ of amain surface of a GaN wafer from the M-plane and a c-axis directioncomponent θc and an a-axis direction component θa of the tilt θ.

FIGS. 12A and 12B are schematic diagrams each illustrating arrangementof an X-ray source, a GaN wafer, and a detector in X-ray rocking curvemeasurement, and FIG. 12A illustrates the arrangement when viewed from adirection parallel to a main surface of the GaN wafer and perpendicularto a plane of incidence of X-rays while FIG. 12B illustrates thearrangement when viewed from a direction perpendicular to the mainsurface of the GaN wafer.

FIG. 13 illustrates a cross-sectional view of a tiling GaN seed in whichangles each formed by the front surface and a side surface are obtuseangles (>90°).

DESCRIPTION OF EMBODIMENTS

In a GaN crystal, a crystal axis parallel to [0001] and [000-1] isreferred to as a c-axis, a crystal axis parallel to <10-10>is referredto as an m-axis, and a crystal axis parallel to <11-20>is referred to asan a-axis. In addition, a crystal plane perpendicular to the c-axis isreferred to as a C-plane, a crystal plane perpendicular to the m-axis isreferred to as an M-plane, and a crystal plane perpendicular to thea-axis is referred to as an A-plane.

Herein, when crystal axes, crystal planes, crystal orientations, and thelike are mentioned, they mean crystal axes, crystal planes, crystalorientations, and the like of a GaN crystal, unless otherwise specified.

A name or Miller indices of a crystal plane which is attached to a nameof a GaN wafer is that of a low-index plane which is parallel or closestto parallel to, out of two main surfaces of the wafer, the main surfacethat is on the side intended to be used for forming a semiconductordevice and/or epitaxially growing a crystal.

For example, a GaN wafer in which a low-index plane parallel or closestto parallel to such a main surface is the M-plane, or in other words,{10-10}, is referred to as an M-plane wafer or a {10-10} wafer.

Usually, a crystal plane for which all of the absolute values ofintegers h, k, m, and 1 of Miller indices {hkml} are smaller than orequal to 3 is assumed to be a low-index plane.

1. First Embodiment

A first embodiment of the present invention relates to a GaN wafer.

FIG. 1 is a perspective view illustrating an example of a GaN waferaccording to the first embodiment. FIG. 2A is a plan view of the GaNwafer 10 illustrated in FIG. 1. FIG. 2B is a cross-sectional view of theGaN wafer 10 illustrated in FIG. 1 and illustrates a cross sectionformed by cutting the wafer along a plane perpendicular to the a-axis.

Referring to FIGS. 1, 2A and 2B, the GaN wafer 10 is an M-plane waferhaving a shape of a disk, and has a first main surface 11 and a secondmain surface 12 which is on a side opposite to the first main surface.The first main surface 11 and the second main surface 12 are connectedto each other via a side surface 13. Preferably, the first main surface11 and the second main surface 12 are parallel to each other.

The first main surface 11 is a main surface intended to be used forforming a nitride semiconductor device and/or epitaxially growing a GaNcrystal, and finished to be a flat surface suitable for these purposes.For example, a root-mean-square (RMS) roughness of the first mainsurface 11 measured by AFM is usually less than 5 nm, preferably lessthan 2 nm, more preferably less than 1 nm in a measured area of 10 μm×10μm.

The first main surface 11 may be tilted relative to the M-plane by amaximum of about 5°. The direction of the tilt is not limited andpreferably such that when the tilt of the first main surface 11 from theM-plane is decomposed into a c-axis direction component and an a-axisdirection component, the absolute value of the c-axis directioncomponent is greater than the absolute value of the a-axis directioncomponent.

The relationship between a tilt of a main surface of a GaN wafer fromthe M-plane and a c-axis direction component and an a-axis directioncomponent of the tilt is as illustrated in FIG. 11.

A tilt of a main surface of a GaN wafer from the M-plane is in otherwords a tilt θ of a normal vector N of the main surface from the m-axis.In order for the tilt θ to be decomposed into a c-axis directioncomponent and an a-axis direction component, the normal vector N isdecomposed into an A-plane parallel component N_(//A), and a C-planeparallel component N_(//C). The A-plane parallel component N_(//A) isthe orthogonal projection of the normal vector N on the A-plane, and theC-plane parallel component N_(//C) is the orthogonal projection of thenormal vector N on the C-plane. The tilt of the A-plane parallelcomponent N_(//A) relative to the m-axis corresponds to a c-axisdirection component θc of the tilt θ, and the tilt of the C-planeparallel component N_(//C) relative to the m-axis corresponds to ana-axis direction component θa of the tilt θ.

In a particularly preferable example, the absolute value of the c-axisdirection component of the tilt of the first main surface relative tothe M-plane is from 2 to 5°, and the absolute value of the a-axisdirection component of the tilt is less than 1°. The c-axis directioncomponent may be either positive or negative, and is preferablynegative. Here, “the c-axis direction component is positive” means thatthe A-plane parallel component N_(//A) of the normal vector of the firstmain surface is tilted relative to the m-axis toward the [0001] side,and “the c-axis direction component is negative” means that the A-planeparallel component N_(//A) of the normal vector of the first mainsurface is tilted relative to the m-axis toward the [000-1] side.

The GaN wafer 10 is thick enough to be handled as a freestanding wafer.In the case of a disk-shaped GaN wafer having a diameter of from 45 to55 mm (about 2 inch), the minimum thickness necessary to allow the waferto be handled as a freestanding wafer is from 150 to 200 μm, a favorablethickness is 250 μm or more, and a further favorable thickness is 280 μmor more. If the wafer has a larger diameter, a favorable thickness isalso larger.

While the thickness of the GaN wafer 10 does not have any particularupper limit, the thickness is usually 1 mm or less, and when thediameter is from 45 to 55 mm, the thickness is preferably 400 μm orless.

On the first main surface 11, the GaN wafer 10 has threelower-crystallinity bands 14 each extending in a direction perpendicularto the c-axis. The lower-crystallinity bands are band-shaped areas eachcomprising a portion in which crystal orientation is disarranged. Darkspot (dislocation) densities on the first main surface 11 are observedto be increased in areas corresponding to the lower-crystallinity bands14 by CL (Cathode Luminescence) measurement.

The presence of the lower-crystallinity bands 14 indicates that a tilingmethod was used in the production process of a GaN crystal forming theGaN wafer 10.

As will be described later, a GaN wafer made of a GaN crystal grown byusing a tiling method has a lower-crystallinity band on a main surfacethereof. In addition, when a GaN wafer having a lower-crystallinity bandon a main surface thereof is used as a seed to grow a GaN crystal, a GaNwafer made from the GaN crystal also has a lower-crystallinity band on amain surface thereof.

A full width at half maximum of X-ray rocking curve (XRC-FWHM), which isan index of crystal orientation, greatly changes at the boundariesbetween a lower-crystallinity band and other areas, and XRC-FWHMs atmany portions within a lower-crystallinity band are greater than thosein areas outside the lower-crystallinity band.

In the GaN wafer 10, the lower-crystallinity bands 14 on the first mainsurface 11 each have a width w of usually 190 μm or less, preferably 150μm or less, more preferably 120 μm or less, more preferably 90 μm orless, more preferably 60 μm or less.

The width of a lower-crystallinity band a GaN wafer has on a mainsurface can be examined by X-ray rocking curve measurement using anX-ray diffractometer equipped with a semiconductor pixel detector (forexample, PIXcel^(3D)® from PANalytical). Arrangement of an X-ray source,a GaN wafer, and a detector in such X-ray rocking curve measurement isschematically illustrated in FIGS. 12A and 12B. FIG. 12A is a drawingillustrating the arrangement when viewed from a direction parallel to amain surface 16 of a GaN wafer 30 and perpendicular to a plane ofincidence of X-rays 405, while FIG. 12B is a drawing illustrating thearrangement when viewed from a direction perpendicular to the mainsurface 16 of the GaN wafer 30.

An X-ray source 400, the GaN wafer 30, and a semiconductor pixeldetector 500 are arranged such that at an intermediate position betweenthe X-ray source 400 and the detector 500, the X-rays 405 are incidenton the main surface 16 of the GaN wafer 30.

The semiconductor pixel detector 500 has a one-dimensional pixel array505 in which n pixels each functioning as an X-ray detector are arrangedside by side in a row at a constant pitch of 2L/n. Here, 2L is thelength of the one-dimensional pixel array 505 (distance between one endand the other end of the array). The one-dimensional pixel array may bepart of a two-dimensional pixel array. The semiconductor pixel detector500 is arranged such that the one-dimensional pixel array isperpendicular to the plane of incidence of the X-rays 405.

By single a scan measurement with the above-described arrangement, it ispossible to at once measure X-ray rocking curves at every L/n on onemeasurement line 17 parallel to the one-dimensional pixel array 505 andhaving a length L which is half the length of the pixel array.

For example, when using a semiconductor pixel detector with aone-dimensional pixel array comprising 256 pixels arranged side by sidein a row at a pitch of 55 μm and having a length of about 14 mm, X-rayrocking curves can be measured at every 27.5 μm on an about 7 mm longmeasurement line parallel to the pixel array in a single a scan.

Thus, by performing an a scan with an X-ray source, a GaN wafer, and asemiconductor pixel detector arranged in such a manner to allow ameasurement line to cross a lower-crystallinity band at a right angle,the width of the lower-crystallinity band at a position where thelower-crystallinity band intersects with the measurement line can beexamined.

Referring again to FIG. 2A, the number of the lower-crystallinity bands14 the GaN wafer 10 has on the first main surface 11 is not limited tothree and may be four or more, but it is desirable not to exceed six.When the GaN wafer 10 has a diameter of from 45 to 55 mm, the number ofthe lower-crystallinity bands 14 is preferably two, more preferably one.

The broken line in FIG. 2A indicates the boundary between areas on thefirst main surface 11, one distant from the outer periphery by less than3 mm and the other distant from the outer periphery by 3 mm or more. Onthe first main surface 11, XRC-FWHMs of the (100) plane measured in anarea circled by the broken line (that is, the area distant from theouter periphery by 3 mm or more) are preferably less than 0.01°, morepreferably 0.008° or less, except in the lower-crystallinity bands 14.

The GaN crystal forming the GaN wafer 10 comprises a GaN crystal grownby an HVPE method and is characterized by having a low alkali metalconcentration and a high transparency in a visible short wavelengthrange. As to alkali metals, each of elements such as Na (sodium) and K(potassium) has a concentration of less than 1×10¹⁵ cm⁻³. As to thetransparency in a visible short wavelength range, an absorptioncoefficient for light with a wavelength of 450 nm is 2 cm⁻¹ or less forexample.

The GaN crystal forming the GaN wafer 10 may contain O (oxygen) at aconcentration of 5×10¹⁷ cm⁻³ or more.

The GaN wafers according to the first embodiment may be variousmodifications of the GaN wafer 10 illustrated in FIGS. 1 and 2.

In one modification, the first main surface of the GaN wafer may have ashape other than a circle and may be shaped into a quadrangle (square,rectangle) for example.

In one modification, the first main surface of the GaN wafer may betilted relative to the M-plane by an angle exceeding 5°. The directionof the tilt is not limited but preferably such that when the tilt of thefirst main surface from the M-plane is decomposed into a c-axisdirection component and an a-axis direction component, the absolutevalue of the a-axis direction component is 5° or less, more preferablyless than 2°, more preferably less than 1°. The smaller the absolutevalue of the a-axis direction component is, the more easily a nitridesemiconductor thin film with a high surface flatness is grown on thefirst main surface 11.

On the other hand, the absolute value of the c-axis direction componentis usually 45° or less, preferably less than 30°, more preferably lessthan 206°. When the absolute value of the c-axis direction componentexceeds 45°, the merit as a semi-polar wafer tends to be lost. Thec-axis direction component may be positive or negative, but preferablynegative.

Preferable examples of the GaN wafer having a first main surface tiltedrelative to the M-plane by an angle exceeding 5° include {10-11} wafers,{10-1-1} wafers, {20-21} wafers, {20-2-1} wafers, {30-31} wafers, and{30-3-1} wafers.

In {10-11} wafers, {20-21} wafers, and {30-31} wafers, the c-axisdirection component of the tilt of the main surface relative to theM-plane is positive, while in {10-1-1} wafers, {20-2-1} wafers, and{30-3-1} wafers, the c-axis direction component of the tilt of the mainsurface relative to the M-plane is negative.

The GaN wafer according to the first embodiment may have, in addition tothe lower-crystallinity band extending in a direction perpendicular tothe c-axis, a lower-crystallinity band extending in a directionperpendicular to the a-axis on the first main surface. FIG. 3 is a planview illustrating one example of such a GaN wafer.

Referring to FIG. 3, a GaN wafer 20 has a shape of a disk and has, inaddition to lower-crystallinity bands 24-1 each extending in a directionperpendicular to the c-axis, a lower-crystallinity band 24-2 extendingin a direction perpendicular to the a-axis on a first main surface 21.Preferably, the GaN wafer 20 has a diameter of from 95 to 155 mm.

The GaN wafer according to the first embodiment may be used as asubstrate for producing various nitride semiconductor devices. Usually,one or more nitride semiconductors are grown by vapor phase epitaxy onthe GaN wafer of the present invention to form a device structure.Preferably usable epitaxial growth methods include MOCVD methods, MBEmethods, and pulse deposition methods, which are suitable for formingthin films.

Specific examples of the nitride semiconductor device include lightemitting devices such as light emitting diodes and laser diodes,electronic devices such as rectifiers, bipolar transistors, field effecttransistors, and HEMTs (High Electron Mobility Transistors),semiconductor sensors such as temperature sensors, pressure sensors,radiation sensors, and visible-ultraviolet light detectors, SAW (SurfaceAcoustic Wave) devices, vibrators, resonators, oscillators, MEMS (MicroElectro Mechanical System) components, voltage actuators, and solarcells.

2. Second Embodiment

A second embodiment of the present invention relates to a method forproducing a non-polar or semi-polar GaN wafer.

The method for producing a non-polar or semi-polar GaN wafer accordingto the second embodiment includes the following steps 1 to 6.

Step 1: growing a GaN crystal in the [000-1] direction on the (000-1)surface of a C-plane GaN seed by an ammonothermal method.

Step 2: making an M-plane GaN seed from the GaN crystal grown in the[000-1] direction in Step 1 above.

Step 3: growing a bulk GaN crystal by an ammonothermal method on theM-plane GaN seed made in Step 2 above.

Step 4: making tiling GaN seeds from the bulk GaN crystal grown in Step3 above.

Step 5: growing a bulk GaN crystal by a tiling method by using thetiling GaN seeds made in Step 4 above.

Step 6: making a non-polar or semi-polar GaN wafer having a desiredsurface orientation from the bulk GaN crystal grown in Step 5 above.

Details of the steps will be described below.

(Step 1)

In Step 1, used as the C-plane GaN seed is a conventional C-plane GaNwafer having a growth mask with a stripe pattern (line and spacepattern) formed on the (000-1) surface (nitrogen polar surface) thereofby using a TiW alloy. The stripe direction of the growth mask isparallel to the a-axis of GaN, and openings of the growth mask each havea width of for example 100 μm. The growth mask is prevented fromcovering the side surface(s) of the C-plane GaN wafer.

As for a crystal growth apparatus and crystal growth conditions used ingrowing a GaN crystal by an ammonothermal method on the C-plane GaNseed, see WO2014/129544 and WO2015/020161.

By using appropriate conditions, on the (000-1) surface of the C-planeGaN seed, a GaN crystal grows at the position of each opening of thegrowth mask in the [000-1] direction to form a wall-like structure inwhich the c-axis direction is defined as the height direction and them-axis direction is defined as the thickness direction.

For example, as a result of growth for a total of 100 days whilereplacing a growth vessel during the growth, the height of the wall (thesize of the grown GaN crystal in the c-axis direction) can reach as highas 20 mm.

The growth of a GaN crystal also occurs on the side surface(s) of theC-plane GaN seed. GaN crystals grow from the entire side surface of theC-plane GaN seed and extend in the [000-1] direction to form aperipheral wall structure enclosing a plurality of the wall-like GaNcrystals growing on the (000-1) surface of the seed.

(Step 2)

The GaN crystal grown in a wall-like shape on the (000-1) surface of theC-plane GaN seed in Step 1 above is removed from the seed and processedto provide an M-plane GaN seed. The M-plane GaN seed is a plate-likecrystal having a main surface substantially parallel to the M-plane.

The main surfaces of the M-plane GaN seed are both subjected toplanarization by lapping, followed by CMP finishing for removal of adamaged layer.

(Step 3)

On the M-plane GaN seed made in Step 2 above, a GaN crystal is grown byan ammonothermal method. The GaN crystal grows in a manner to cover anentire surface of the M-plane GaN seed. The growth direction of the GaNcrystal on the main surface of the M-plane GaN seed is the m-axisdirection.

(Step 4)

The GaN crystal grown in Step 3 above is sliced parallel to the M-plane,and edge portions of the obtained GaN crystal plate are cut off with adicing saw to thereby make a tiling GaN seed which has rectangular mainsurfaces with long sides perpendicular to the c-axis and short sidesperpendicular to the a-axis.

By adjusting the angle of slicing, the main surface of the tiling GaNseed may be slightly tilted from the M-plane.

Side surfaces of the tiling GaN seed are cut surfaces which are formedby cutting the GaN crystal plate with the dicing saw. A deviation oforientation of the side surface of the tiling GaN seed from a designedorientation can be kept within 0.1° by repeating the followingoperation: confirming, by an X-ray diffraction method, an orientation ofa formed cut surface after every cutting; if a deviation from a designedorientation exceeds 0.1°, adjusting the direction of the work; and onceagain cutting the work.

When cutting the GaN crystal plate with the dicing saw, it is desirableto apply the saw blade from the side that serves as a back surface insubsequent Step 5. The back surface refers to the main surface on theopposite side of a front surface, when the front surface is the mainsurface on the side to be used for epitaxially growing a bulk GaNcrystal among the two main surfaces of the tiling GaN seed (the sameapplies hereinafter).

The reason is for preventing the front surface and a side surface fromforming an obtuse angle in the tiling GaN seed. Since a dicing saw has ablade which decreases in thickness toward the tip, when the blade isapplied from the front surface side to cut the tiling GaN seed, an angleformed by the front surface and the side surface (cut surface) tends tobe an obtuse angle (>90°) as shown in FIG. 13 in a cross-sectional view.

By setting the angle formed by the front surface and the side surface tobe a right or acute angle, a gap between the front surfaces of adjacenttiling GaN seeds can be prevented from occurring when a plurality of thetiling GaN seeds are closely arranged side by side.

Usually, both main surfaces of the tiling GaN seed are subjected toplanarization processing. Specifically, after grinding and/or lappingare performed, a damaged layer is removed by CMP.

The planarization processing is performed in the order of the backsurface first, the front surface later (as described above, the frontsurface refers to the main surface on the side to be used forepitaxially growing a bulk GaN crystal in subsequent Step 5, and theback surface refers to the main surface on the opposite side). Inparticular, in planarization processing of the front surface, as shownin FIG. 4A in a cross-sectional view, a plurality of seeds 100 are fixedon a flat surface of a plate P while being closely arranged side by side(the back surfaces of the seeds are adhered to the plate P). Asillustrated in FIG. 4B in a cross-sectional view, this allows, among thetiling GaN seeds 100 after planarization processing, variation inthickness to be extremely small, while suppressing edge roll-off ofprocessed surfaces (front surfaces) in portions where the seeds are incontact with each other.

In particular, when the angle formed by the front surface and the sidesurface of the tiling GaN seed is set so as not to be an obtuse angle,in closely arranging side by side a plurality of the seeds andsubjecting their front surfaces to planarization processing, a gapbetween the front surfaces of adjacent seeds is allowed to be small, andtherefore the edge roll-off suppressing effect is preferably exerted.

(Step 5)

The plurality of tiling GaN seed each having the front surface that hasbeen planarized while being closely arranged side by side on the flatsurface of the plate, are closely arranged side by side on a susceptorof an HVPE apparatus in the same arrangement as when fixed on the plate,with the front surface facing upward, to thereby form an aggregatedseed. On the aggregated seed, a bulk GaN crystal is epitaxially grown byan HVPE method (tiling method). It is preferable that carrier gassupplied to a reactor during growth of the bulk GaN crystal is nitrogengas only.

FIG. 5 illustrates a cross sectional view of a composite of anaggregated seed 200 and a bulk GaN crystal 300 resulting from theepitaxial growth.

While the bulk GaN crystal 300 contains a lower-crystallinity portion305 above the boundary between adjacent tiling GaN seeds 100, thethickness t of the portion is small, because the difference in surfaceorientation of the front surfaces of two tiling GaN seeds in thevicinity of the boundary is small. This is due to suppression of edgeroll-off of the front surfaces in the vicinity of the boundary bysubjecting the front surfaces to the planarization processing in themanner as described in Step 4.

In addition, for reducing the thickness of the lower-crystallinityportion caused in the bulk GaN crystal, it is also preferable to preventthe front surface and the side surface to form an obtuse angle in thetiling GaN seed, because the gap between the front surfaces of adjacentseeds in the aggregated seed is thereby allowed to be extremely small.

For comparison, a case will be described in which a plurality of tilingGaN seeds are fixed to a surface of a plate while being spaced apartfrom each other, and subjected to planarization processing of frontsurfaces. In this case, as illustrated in a cross-sectional view in FIG.6, edge roll-off of the front surfaces of tiling GaN seeds 1000 is notsuppressed.

FIG. 7 illustrates a cross-sectional view of a composite of anaggregated seed and a bulk GaN crystal epitaxially grown thereon whichis obtained when the aggregated seed is configured with such tiling GaNseeds each having a large edge roll-off amount and a tiling method isperformed.

Referring to FIG. 7, the bulk GaN crystal 3000 grown on the aggregatedseed 2000 made of a plurality of the tiling GaN seeds 1000 containsthick lower-crystallinity portions 3005 above the boundaries betweenadjacent tiling GaN seeds.

(Step 6)

By slicing the bulk GaN crystal obtained in Step 5 above, a non-polar orsemi-polar GaN wafer is obtained. The direction of slicing may beparallel to or may be tilted relative to the front surfaces of thetiling GaN seeds.

Main surfaces of the GaN wafer are subjected to planarizationprocessing. Specifically, after grinding and/or lapping are performed, adamaged layer is removed by CMP.

For example, by processing the composite illustrated in FIG. 5 into acylindrical shape through outer periphery grinding or core drilling andsubsequently slicing the processed composite at positions indicated bybroken lines in FIG. 8, the GaN wafer 10 illustrated in FIG. 1 isobtained. The lower-crystallinity portions 305 in the bulk GaN crystal300 are thereby exposed on the main surface 11 of the GaN wafer 10 andform the lower-crystallinity bands 14.

3. Experimental Results 3.1. EXAMPLE

A C-plane GaN wafer was prepared, and on its (000-1) surface (nitrogenpolar surface), a growth mask with a stripe pattern was formed by usinga TiW alloy. The stripe direction was parallel to the a-axis of GaN, andthe openings each had a width of 100 μm.

On the (000-1) surface of the C-plane GaN wafer having such a growthmask formed thereon, a GaN crystal was grown by an ammonothermal method.Polycrystalline GaN was used as a feedstock, and ammonium fluoride(NH₄F) and ammonium iodide (NH₄I) were used as mineralizers.

Amounts of charge of the mineralizers were determined so that NH₄F andNH₄I were from 0.5 to 1.5% and from 1.5 to 3.5%, respectively, in termsof molar ratio to NH₃ sealed in a growth vessel and that molar ratio ofNH₄F to NH₄I was from 0.2 to 0.5.

Growth conditions were as follows: the average temperature in the growthvessel (the average value of the temperature in a crystal growth zoneand the temperature in a feedstock dissolution zone) was from 590 to630° C.; the temperature difference between the crystal growth zone andthe feedstock dissolution zone was from 5 to 20° C.; and the pressureinside the growth vessel was from 200 to 220 MPa.

As a result of growth for a total of 100 days while replacing the growthvessel twice during the growth, at the position of each opening of thegrowth mask, a wall-like GaN crystal in which the c-axis direction wasdefined as the height direction and the m-axis direction was defined asthe thickness direction was grown.

The GaN crystal grown like a wall was removed from the C-plane GaN waferand processed into a flat plate having a main surface substantiallyparallel to the M-plane (M-plane GaN seed). The main surfaces of theM-plane GaN seed were both subjected to planarization by lapping,followed by CMP finishing for removal of a damaged layer.

Next, on this M-plane GaN seed, a GaN crystal was grown by anammonothermal method again. In this second ammonothermal growth, amountsof charge of the mineralizers were set so that fluorine atoms and iodineatoms were 0.5% and 1.5%, respectively, in terms of molar ratio to NH₃.The average temperature in the growth vessel was from 600 to 611° C.,and the temperature difference between the crystal growth zone and thefeedstock dissolution zone was from 9 to 13° C. The pressure inside thegrowth vessel was the same as that in the first ammonothermal growth.

From the GaN crystal grown by the second ammonothermal growth on theM-plane GaN seed, a tiling GaN seed having a plate-shape and a thicknessof about 330 μm was made in the following procedure.

First, the GaN crystal was sliced parallel to the M-plane with amulti-wire saw. Subsequently, edge portions of a GaN crystal platesliced were cut off with a dicing saw to shape the main surface of theGaN crystal plate into a rectangle with long sides parallel to thea-axis and short sides perpendicular to the a-axis. In cutting with thedicing saw, the saw blade was always applied to the GaN crystal platefrom the back surface side. For any of the long sides and the shortsides of the rectangle, a deviation from a designed orientation was setto 0.1° or less.

Finally, each main surface of the GaN crystal plate was subjected toplanarization processing to complete a tiling GaN seed. Specifically,after grinding and/or lapping were performed, a damaged layer wasremoved by CMP.

The order of the planarization processing was the back surface first,the front surface later. The front surface is the main surface on theside to be used later for epitaxially growing a bulk GaN crystal by anHVPE method.

The planarization processing of the front surface was carried out in astate where five GaN crystal plates were adhered with wax to a flatsurface of a plate while being closely arranged side by side in thec-axis direction. More specifically, the five GaN crystal plates werealigned in a row on the flat surface of the plate such that between anytwo neighboring GaN crystal plates, the [0001] side end of one plate andthe [000-1] side end of the other plate are in contact with each other,thereby making the [0001] directions of all the five GaN crystal platesidentical.

Next, the five tiling GaN seeds, which had been obtained by planarizingtheir front surfaces in the state where they were arranged closely sideby side, were closely arranged on a susceptor of an HVPE apparatus, withthe front surface facing upward, to thereby form an aggregated seed. Thearrangement of the five seeds in the aggregated seed was the same asthat in the planarization processing of the front surfaces.

On this aggregated seed, a bulk GaN crystal was epitaxially grown by anHVPE method. The growth temperature was 1050° C., and the growth timewas 82 hours. Carrier gas supplied to a reactor during growth of thebulk GaN crystal was nitrogen gas only.

The grown bulk GaN crystal was processed into a cylindrical shapetogether with the seeds, and subsequently sliced into wafers. Thedirection of slicing was adjusted such that the main surface of thewafer was tilted relative to the M-pane by 5° in the [000-1] direction.After slicing, both main surfaces of the wafer were subjected toplanarization processing to thereby provide an off-angled M-plane GaNwafer having a diameter of 50 mm and a thickness of 280 μm and havingfour lower-crystallinity bands each extending on the main surface in adirection perpendicular to the c-axis.

Among the off-angled M-plane GaN wafers of the Example made followingthe above-described procedure, one wafer was selected, and X-rays (CuKαrays) were allowed to enter the main surface of the selected wafer tomeasurefull width at half maximums of X-ray rocking curve (XRC-FWHMs) ofthe (100) plane. The selected wafer was cut out from the bulk GaNcrystal at a part about 5 mm distant from the seed surfaces.

The measurement was carried out using a high-resolution X-raydiffractometer (PAnalytical X'Pert PRO MRD) equipped with an X-raymirror and a Ge (220) 2-crystal hybrid monochromator in an incidentoptics. The incident direction of X-rays was perpendicular to the c-axisof the M-plane GaN wafer. A 1/32 divergence slit and a 0.05 mm verticallimitation slit were used on the incidence side.

By capturing intensity data with the use of a semiconductor pixeldetector for an co range of 0.4° at every step of 0.001° for anaccumulated time of 10 seconds, it was possible to obtain an XRC-FWHM atabout every 27 μm on a measurement line extending on the main surface ofthe wafer in the c-axis direction and having a length of about 7 mm. Thesemiconductor pixel detector used was PIXcel^(3D)® from PANalytical andconfigured to include a pixel array in which 256 pixels were arrangedside by side in a row at a pitch of 55 μm.

FIGS. 9A to 9C each illustrate XRC-FWHMs obtained at about every 27 μmon a measurement line selected to cross a lower-crystallinity band at aright angle. FIGS. 9A to 9C illustrate respective results obtained ondifferent measurement lines. In FIGS. 9A to 9C, the horizontal axesindicate relative positions on the measurement line (unit: mm), whilethe vertical axes indicate XRC-FWHMs (unit:)° on the (100) plane.

In all of FIGS. 9A to 9C, XRC-FWHMs on the measurement lines were 0.008°or less (approximately within a range of from 0.006 to 0.007°, except insections indicated as “lower-crystallinity section” in the graphs. Inthe lower-crystallinity sections, XRC-FWHMs locally increase to 0.01° ormore.

Separately performed CL image observation showed that respectivelower-crystallinity sections in FIGS. 9A to 9C each corresponded to aportion where the measurement line crossed the lower-crystallinity band.It can therefore be said that the section length of thelower-crystallinity section is the width of the lower-crystallinityband.

In each of FIGS. 9A to 9C, the lower-crystallinity section had a sectionlength of less than 60 μm.

3.2 Comparative Example

In a Comparative Example, the following four points were changed fromthe Example.

Firstly, the number of the tiling GaN seeds used was four.

Secondly, in subjecting the front surfaces of the four tiling GaN seedsto planarization processing, the four seeds were fixed to the surface ofthe plate while being spaced apart from each other.

Thirdly, growth time of the bulk GaN crystal by the HVPE method was 61hours.

Fourthly, in slicing the bulk GaN crystal, the direction of slicing wasadjusted such that the main surface of the wafer was tilted relative tothe M-plane by 2° in the [000-1] direction.

Except for the above four points of change, off-angled M-plane GaNwafers of the Comparative Example were made in the same manner as in theExample.

One wafer was selected from the wafers made, and by using the samemeasurement method as in the Example, XRC-FWHMs on the (100) plane atabout every 27 μm on a measurement line extending on the main surface ofthe wafer in the c-axis direction was obtained. The selected wafer wascut out from the bulk GaN crystal at a part about 0.2 mm distant fromthe seed surfaces.

FIGS. 10A to 10C each illustrate XRC-FWHMs on the (100) plane obtainedat about every 27 μm on a measurement line selected to cross alower-crystallinity band at a right angle. FIGS. 10A to 10C illustraterespective results obtained on different measurement lines. In FIGS. 10Ato 10C, the horizontal axes and the vertical axes indicate the samevalues as in FIGS. 9A to 9C.

In all of FIGS. 10A to 10c, XRC-FWHMs on the measurement lines were0.008° or less (approximately within a range of from 0.006 to 0.007°,except in sections indicated as “lower-crystallinity section” in thegraphs. The lower-crystallinity sections include portions whereXRC-FWHMs increased to 0.01° or more.

Separately performed CL image observation showed that respectivelower-crystallinity sections in FIGS. 10A to 10C each corresponded to aportion where the measurement line crossed the lower-crystallinity band.The lower-crystallinity section had a section length of 660 μm in FIG.10A, 1070 μm in FIGS. 10 B, and 190 μm in FIG. 10C.

In FIGS. 10A to 10C, even in the lower-crystallinity sections, portionswhere XRC-FWHMs are less than 0.01° are locally found. In other words,in lower-crystallinity bands, small areas where crystal orientation isnot disarranged exist in some places. However, since such small areasare mixed with areas where crystal orientation is significantlydisarranged, it is considered difficult to effectively utilize suchsmall areas for forming semiconductor devices.

Although the present invention has been described with reference tospecific embodiments as above, each embodiment was presented as anexample and does not limit the scope of the present invention. That isto say, each of the embodiments described herein can be variouslymodified without departing from the spirit of the invention, and can becombined with characteristics described by other embodiments so long asit can be enabled.

REFERENCE SIGNS LIST

10, 20, 30 GaN wafer

11, 21 first main surface

12 second main surface

13, 23 side surface

14, 24-1, 24-2 lower-crystallinity band

16 main surface

17 measurement line

100, 1000 tiling GaN seed

200, 2000 aggregated seed

300, 3000 bulk GaN crystal

15, 305, 3005 lower-crystallinity portion

400 X-ray source

405 X-rays

500 semiconductor pixel detector

505 one-dimensional pixel array

P plate

1. A GaN wafer comprising: a first main surface; a second main surfaceon a side opposite to the first main surface; and a lower-crystallinityband extending on the first main surface in a direction perpendicular toa c-axis, wherein the first main surface is parallel to or tiltedrelative to an M-plane, when the tilt, if exists, is decomposed into ana-axis direction component and a c-axis direction component, the a-axisdirection component has an absolute value of 5° or less while the c-axisdirection component has an absolute value of 45° or less, and thelower-crystallinity band has a width of less than 190 μm.
 2. The GaNwafer according to claim 1, wherein the lower-crystallinity band has awidth of less than 150 μm.
 3. The GaN wafer according to claim 2,wherein the lower-crystallinity band has a width of less than 120 μm. 4.The GaN wafer according to claim 1, wherein X-ray rocking curve fullwidth at half maximums of a (100) plane measured in an area distant froman outer periphery by 3 mm or more on the first main surface are lessthan 0.01°, except in the lower-crystallinity band.
 5. The GaN waferaccording to claim 1, wherein the GaN wafer is a {10-10} wafer, a{10-11} wafer, a {10-1-1} wafer, a {20-21} wafer, a {20-2-1} wafer, a{30-31} wafer or a {30-3-1} wafer.
 6. The GaN wafer according to claim1, wherein the GaN wafer is a disk having a diameter of from 45 to 55mm, and the number of the lower-crystallinity band the GaN wafer has onthe first main surface is one or greater and three or smaller.
 7. TheGaN wafer according to claim 1, further comprising a lower-crystallinityband extending on the first main surface in a direction perpendicular toan a-axis.
 8. The GaN wafer according to claim 7, wherein the GaN waferis a disk having a diameter of from 95 to 155 mm.
 9. A method forproducing a nitride semiconductor device, comprising: preparing the GaNwafer according to claim 1; and growing one or more nitridesemiconductors on the prepared GaN wafer by vapor phase epitaxy to forma device structure.